In general, as a technique of controlling the sensitivity of a developer in an automatic developing apparatus for photosensitive lithographic printing plates, there is adopted a time- and treatment-based replenishing method of a developer replenisher (hereinafter, referred to as an “area- and time-based replenishing method”) in which a developer tank storing a developer therein is replenished with a developer replenisher with time, and the plate area of a lithographic printing plate to be treated is measured, so that the tank is replenished with the developer replenisher in an amount corresponding to the measured value.
However, with such an area- and time-based replenishing method, a plate area measuring device of a lithographic printing plate becomes necessary at a development treatment unit of the automatic developing apparatus, unfavorably resulting in a more complicated structure, and incurring a high cost. Further, it is difficult to determine whether the lithographic printing plate has only a single-sided or double-sided photosensitive layers (hereinafter, referred to as “single sided/double sided”), and to determine the plate types (of the plates mutually different in coating amount of the photosensitive layer, or the like). For this reason, when the required replenishing amount of the developer replenisher is changed due to the changes in the plate area, single sided/double sided, or the plate type of the lithographic printing plate, unfavorably, it becomes difficult to properly carry out the supply of the developer replenisher.
Under such circumstances, conventionally, for example, as described in Japanese Patent No. 2516022 (JP-A-64-21451; the term “JP-A” as used herein means an “unexamined published Japanese patent application), as a developer replenisher supply method of an automatic developing apparatus for photosensitive lithographic printing plates, there has been adopted a replenishing method (hereinafter, referred to as an “electric conductivity-based replenishing method”) as follows: the electric conductivity of the developer is measured, and the measured value is compared with the electric conductivity value experimentally determined in advance, indicative of the optimum sensitivity (hereinafter, referred to as a “proper electric conductivity value”); when the measured value falls short of the proper electric conductivity value, the developer replenisher is supplied. With such an electric conductivity-based replenishing method, even when the plate area, single sided/double sided, or the plate type of the lithographic printing plate to be treated changes, it is possible to carry out the supply of the developer replenisher in a proper amount, and it is possible to keep the sensitivity of the developer proper.
However, in general, there is the empirical fact that the developer fatigued with only a carbonic acid gas over time and the developer fatigued with only the plate treatment have mutually different proper electric conductivity values (silicate type treating agent/when recovered by replenishment from fatigue over time (carbonic acid gas fatigue): 65 mS/cm, when recovered by replenishment from treatment fatigue: 55 mS/cm, non-silicate type treating agent/when recovered by replenishment from fatigue over time: 56 mS/cm, when recovered by replenishment from treatment fatigue: 39 mS/cm). For this reason, with the foregoing electric conductivity-based replenishing method, when the treatment frequency, for example, the treatment amount per day is different from the expected amount, there is also a discrepancy between the ratio of the actual time-based replenishing amount and the treatment-based replenishing amount and the expected value. Therefore, the proper electric conductivity value of the developer calculated from each proper electric conductivity changes. As a result, a discrepancy from the preset proper electric conductivity value is caused, so that unfavorably it becomes impossible to keep the developer sensitivity proper.
In contrast, JP-A-9-96910 proposes an automatic developing apparatus as follows. The replenishment time interval is measured, and the time-based replenishing amount considered to be required within the time interval is determined. The determined time-based replenishing amount is subtracted from the replenishing amount per step. The calculated value is taken as a treatment-based replenishing amount. Thus, the treatment-based replenishing amount and the time-based replenishing amount are respectively determined. Then, the ratio between the obtained treatment-based replenishing amount and time-based replenishing amount is calculated to correct the target electric conductivity value. With this technique, even when the plate area, single sided/double sided, or the plate type of a lithographic printing plate to be treated changes, it is possible to identify the proper target electric conductivity value with precision by estimating the ratio between the treatment-based replenishing amount and the time-based replenishing amount.
However, with the foregoing method for determining the ratio between the treatment-based replenishing amount and the time-based replenishing amount from the replenishment time interval, and correcting the target electric conductivity value, there occurs a problem that it is not possible to determine the precise ratio between the treatment-based replenishing amount and the time-based replenishing amount in the case where the aging exhaustion conditions are different. The case where the time-based conditions are different includes, for example, the case where the carbonic acid gas concentration of the ambient atmosphere has changed, or the case where the reactivity of the carbonic acid gas with a developer has changed. The following facts are known. For the former case, the value changes depending upon the number of workers in a room where an automatic developing apparatus is set. Whereas, for the latter case, it changes depending upon whether the automatic developing apparatus is operating or stopping. In other words, with the foregoing method, a discrepancy arises between the experimentally predetermined time-based replenishing amount on a unit time basis and the actual time-based replenishing amount on a unit time basis depending upon the fluctuations in number of indoor workers, or depending upon whether the automatic developing apparatus has been operating or stopping. For this reason, it is not possible to determine the precise time-based replenishing amount from the replenishment time interval. This causes a fear that an erroneous target electric conductivity value is calculated.
In view of such circumstances, it is therefore an object of the invention to provide a developer replenishing method of an automatic developing apparatus for photosensitive lithographic printing plates, capable of minimizing the fluctuations in developer sensitivity with respect to the changes in development treatment conditions while implementing a developing unit of the automatic developing apparatus in a simple and low cost structure in an electric conductivity-based replenishing method.